High-performance pneumatic tires for two-wheeled vehicles, which are subjected to high-speed rotation, are greatly affected by centrifugal force. This causes outward expansion of the tires at treads thereof in the tire radial direction. Consequently, steering stability performance may be degraded. For such a reason, reinforced structures have been developed in which a reinforcing member is spirally wound around the crown of a tire thereof in such a manner as to be substantially parallel to the tire equatorial plane so that the expansion at the tread is suppressed. Examples of such a reinforcing member spirally wound parallel to the tire equatorial plane include nylon fiber, aromatic polyamide fiber (Kevlar: a trade name), steel and the like. Particularly, aromatic polyamide fiber and steel, which have the characteristic of not expanding easily even at high temperature, can suppress the expansion at the tread and therefore are attracting attention as effective materials for high-performance tires. By winding the reinforcing member around the crown of a tire, a so-called “hoop” effect can be enhanced. Thus, a deformation in which the tire tends to expand at the crown thereof in the tire circumferential direction because of a centrifugal force applied during high-speed rotation can be suppressed. Therefore, many patent applications (Patent Documents 1 to 5, for example) have been made in which such a reinforcing member (hereinafter referred to as a “spiral member”) is spirally wound around the crown of a tire.
It is known that tires having these spiral members wound therearound (tires having spiral belts) have excellent steering stability performance during high-speed traveling and very high traction. However, even with a tire having a spiral member wound therearound, the steering stability performance during turning in which a vehicle (motorcycle) is significantly tilted is not necessarily improved markedly. Moreover, some users and racing riders have a demand for an increase in gripping force during turning in which a motorcycle is significantly tilted.
In response to this, Patent Document 6, for example, discloses a pneumatic tire for two-wheeled vehicles. The tread of this tire, having a spiral belt structure, is divided into three parts (a central region and two side regions of the tread) in the tire axial direction. Tread rubbers for the two side regions have a JIS(A) hardness and a ratio of loss tangent to the dynamic complex modulus of elasticity larger than those of a tread rubber for the central region. With this structure, the lateral rigidity of the spiral structure, which is originally low, is enhanced. Further, since sufficient gripping force during turning is ensured, cornering force and camber thrust are improved, whereby excellent turning performance can be obtained.
In a pneumatic tire for two-wheeled vehicles disclosed in Patent Document 7, a tire tread provided with a circumferential belt layer (equivalent of the spiral belt layer herein) is divided into a central region whose width is 0.2 to 0.4 times a stretched tread width and whose center coincides with the center of the tire tread width, and shoulder regions on both sides of the central region. These regions have different moduli at 100% elongation at least at the grounding surfaces thereof at room temperature. In this case, when the aforementioned moduli of the shoulder regions are set larger than that of the central region, vibration absorbability during linear traveling can be improved and occurrence of shimmy can be prevented, whereby linear traveling stability can be ensured. When the moduli of the shoulder regions are set smaller than that of the central region, high-speed durability and steerability can be improved.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-067059    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-067058    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-011614    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-316512    Patent Document 5: Japanese Unexamined Patent Application Publication No. 09-226319    Patent Document 6: Japanese Unexamined Patent Application Publication No. 07-108805    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2000-158910